This invention is generally directed to piezoelectric speakers and more specifically to protecting same from failures due to overheating.
Piezoelectric speakers have response characteristics that differ substantially from conventional electromagnetic speakers Voice range and tweeter piezoelectric speakers have high frequency response characteristics that extend well beyond 20 kilohertz (kHz). Thus, any energy above the range of human hearing can contribute additional heat build-up in the piezoelectric driving element.
Excessive energy above the range of human hearing may be delivered to a speaker when the audio power amplifier stage is driven beyond its linear power handling capabilities and goes into a nonlinear or "clipping" region. Such action produces harmonics and other nonlinear signals above 20 kHz which contributes to an undesired heating of a piezoelectric speaker.
It is also possible for very high power audio amplifiers which are not driven into a nonlinear region to provide an amount of power beyond the power handling capability of the piezoelectric speaker. This produces excessive thermal heating in the piezoelectric driving element.
Conventional electromagnetic speakers do not encounter the same heating problems due to high frequency energy above 20 kHz. This is because these speakers appear somewhat inductive and thus have an impedance which increases as the frequency increases. The higher impedances at the higher frequencies tend to limit the power accepted at the high frequencies. Of course, high levels of energy below 20 kHz can lead to thermal problems in electromagnetic speakers. In order to limit the power delivered to electromagnetic speakers, various elements have bee connected in series with the speaker including nonlinear resistors.
The excessive heat dissipation problem encountered by piezoelectric speakers has been addressed by using a series resistor Parallel zener diodes connected back-to-back in series with a resistor have been connected in parallel across the speaker terminals in order to limit the voltage which can appear across the speaker. Although the zener diode combination is effective in limiting the voltage and hence energy which can be applied to a piezoelectric speaker, it significantly degrades the audio performance of the speaker since even infrequent high volume passages will be limited. Normally, intermittent high volume passages will not adversely effect a piezoelectric speaker since significant heat in the piezoelectric driver element will not develop. Excessive thermal overloading of a piezoelectric speaker results in irreversible damage and often total failure.
It is an object of the present invention to provide a piezoelectric speaker with improved thermal protection which minimizes audio quality degradation .